Although the capability hasn't rolled out to all users, it hints at an increased threat to one of Facebook's fastest growing businesses.

Several years ago, Facebook launched mobile app install ads, which make it easy for consumers to discover and download new apps.

App install ads appear directly in Facebook users' newsfeeds and provide a link that lets users quickly install a new mobile app. The ads have become very popular among app developers looking for ways to drive downloads and make their apps stand out from the crowd.

But if Google makes developers' apps easier to download directly through its web search engine, they may be less likely to pay for Facebook's ads.

That could pose a problem for Facebook, which has benefited from robust growth in its mobile ad business. Analysts believe the app install ads are an important part of Facebook's broader mobile ad business, although the company has refused to say how much of its ad revenue comes from them.

App search could become a big business

On Google's end, this new roll-out follows a bunch of other ways that Google is making search more app-friendly.

Right now, Google's download-without-Play test only works through organic search, not ads, but it's easy to imagine that extension happening in the future.

BI Intelligence research shows that the mobile app-install ad revenue will likely grow to $6.8 billion by the end of 2019. Although Google's increased focus on integrating app installs into search doesn't mean doom for Facebook (and this capability in particular would likely never apply to Apple's app store), improvements like this still help the company gain mindshare as Android developers think about how to allocate their ad budgets, which could help it carve a bigger piece of that $6.8 billion pie.

Facebook's main argument for why its mobile app install ads are so powerful for developers is that they can target the audience based on Facebook's information about users. Through Google's new quick-download capabilities, developers would have the added benefit: They know when users are searching for their apps.